The present invention relates to an image forming apparatus, and, more particularly, to an image forming apparatus, such as an light emitting diode (LED) printer head, which forms an image on a photosensitive surface by forming the image of light information from an LED array having a plurality of point light sources by a lens array.
A conventional LED printer head includes an LED array having a plurality of LEDs and a lens array which forms an image on a photosensitive surface by forming the image of light information irradiated from the LED array. The lens array comprises two rows of a plurality of gradient index rod lenses. Each rod lens forms the image of light information within a limited range. The lens array forms a total image by overlapping images formed by the lenses.
As shown in FIG. 8, the conventional LED printer head had to adjust the positions of the LED array and a lens array 110 in such a way that LEDs 100 would be positioned on the median plane C of the two rows of rod lenses. For example, Japanese Laid-Open Patent Publication No. 10-309826 discloses an image forming apparatus which is so designed as not to be easily influenced by the mounting errors of the LED array and the lens array in order to eliminate the troublesome position adjustment.
If the resolution of the lens array in an LED printer head which forms an image by causing a plurality of LEDs to emit light in various patterns differs at various locations, i.e., if the resolution of the lens array has a large variation, a linear irregularity occurs in the amount of light. The irregular amount of light results in the formation of uneven point images on the image forming surface, which makes the amount of toner adhered uneven, thereby resulting in uneven printing. A variation in the amounts of lights from the LEDs can be adjusted by compensating for the amount of light from each LED based on the light amount distribution of the surface of an image that has been measured in advance. Because a variation in the resolution of the lens array is a variation in a light amount profile (light amount distribution of a point image), however, the variation cannot be corrected by changing the brightness of the light sources. It is therefore difficult to compensate for a variation in the resolution of the lens array.